Double wall storage tank having an outer jacket bonded around an aperture

ABSTRACT

A multiple wall tank for the storage of liquids is manufactured from a rigid inner tank having at least one aperture by applying a layer of a synthetic resin material to the inner tank exterior surface around the aperture, providing a spacing means to provide for substantially free passage of liquids along at least a portion of the inner tank, applying over the layer of synthetic resin and the inner tank an outer sheath of a thermoplastic synthetic resin material, and bonding the outer sheath to the resin material layer surrounding and proximal to the aperture to form a substantially liquid-impervious seal around the aperture.

RELATED APPLICATIONS

This a continuation-in-part of U.S. patent application, Ser. No.08/735,610, entitled A DOUBLE WALL STORAGE TANK HAVING AN EXTRUDED OUTERSHEATH AND A METHOD FOR MAKING SAME, filed Oct. 23, 1996, and now U.S.Pat. No. 5,816,435.

FIELD OF INVENTION

This invention relates to tanks for the storage of liquids, and moreparticularly to multiple wall tanks for the storage of liquids. Evenmore specifically, the invention relates to a double wall storage tankhaving an outer sheath of a thermoplastic synthetic resin materialbonded around an inner tank aperture and a method for making same.

BACKGROUND OF THE INVENTION

Tanks for the storage of liquids have been constructed in a variety ofways from a variety of materials. In one common application, the storageof hydrocarbons, such as gasoline and other petroleum products, thetanks have conventionally been fabricated out of steel or fiberglass,most commonly with a single rigid wall. In many applications, thisconstruction has proved reasonably satisfactory, with such tanksfunctioning properly for many years before requiring repair orreplacement. However, the increasing age of many of the tanks currentlyin place is beginning to present serious environmental dangers. Many ofthe older steel tanks buried underground have rusted and are beginningto leak, thus releasing the petroleum materials into the ground wherethey may seep into and pollute underground water supplies. Whilerust-proof, some fiberglass tanks have also exhibited leakage, causingsimilar problems.

One of the primary problems with leaking storage tanks has been thedifficulty or inability to ascertain when or if such leaks are occurringfrom a given tank. Because the excavation and removal of such a storagetank, which may contain thousands of gallons of fuel, is an expensiveand difficult undertaking, such an operation is difficult to justifyunless there is some evidence of actual leakage.

Because of the increasing potential danger of leaking storage tanks,particularly in communities that utilize ground water for publicconsumption, many municipalities have implemented or plan to implementordinances requiring the use of double wall storage tanks undergroundand requiring replacement of existing single wall tanks. While theinstallation of a conventional double wall tank in a new facilityentails no great difficulty and a generally manageable increase in costover a single wall tank, a heavy burden exists for complying with suchordinances by replacing existing sound, single wall tanks with doublewall tanks. This burden has prompted the search for methods offabricating relatively inexpensive double wall or multi-wall tanks. Thisburden has also given impetus to the search for a method ofremanufacturing existing single wall tanks into double or multi-wallassemblies with means for detecting the presence of any leaks into thespace between the walls.

Several methods for manufacturing double wall tank assemblies have beendeveloped, including that disclosed by David T. Palazzo, the inventor ofthe present invention, in U.S. Pat. No. 4,640,439 and its progeny. Thispatent generally discloses applying a spacing material over a rigidinner tank and bonding resin impregnated glass fiber mats to theexterior surface of spacing material to form a double wall tank. Whileconventional glass fiber provides a relatively effective and inexpensiveouter sheath, other materials, such as polyolefins, exist that mayprovide a convenient alternative to forming a substantially liquidimpervious outer shell around the rigid inner tank.

A thermoplastic synthetic resin material, such as polyethylene, has beenextruded onto the surfaces of steel pipes as disclosed in U.S. Pat. No.4,478,200 to Hakert, et al., and U.S. Pat. No. 3,616,006 to Landgraf, etal. However, these materials ordinarily will not bond adequately tometallic or steel surfaces, with or without adhesives, as is taught bythese and other references. When fabricating a multiple wall storagetank, it is often desired, if not essential, to bond an outer jacket tothe inner tank, which is typically formed of steel, around the fittingsor apertures that may be formed through many conventional single walltanks in order to prevent leakage around these fittings.

SUMMARY OF THE INVENTION

In view of the foregoing, it is an object of the present invention toprovide an economical method of manufacturing a multiple wall storagetank from a rigid, single wall tank. It is a further object of theinvention to provide a method for manufacturing a multiple wall storagetank having a generally rigid, liquid-impervious outer sheath bondedaround fittings or apertures, which may be formed through theconventional inner tank. Still another object of the present inventionis to provide an efficient and relatively simple method of manufacturinga multiple wall storage tank from a rigid, single wall tank.

To achieve these and other objects that will become readily apparent tothose skilled in the art, this invention provides a double wall tank forthe storage of liquids and a method of manufacturing a rigid multiplewall tank for the storage of liquid from a single wall inner tank. Thedouble wall tank of the present invention includes a rigid inner tankhaving generally cylindrical sidewall portions, end portions generallytransverse to the sidewall portions and a central axis extendinggenerally longitudinally between the end portions, with the inner tankhaving an exterior surface and at least one aperture formed through theinner tank. An outer sheath of a substantially liquid-imperviousthermoplastic synthetic resin material surrounds the inner tank, and alayer of a first thermoplastic synthetic resin material is bondedbetween the inner tank and the outer sheath surrounding and proximal tothe aperture, such that a substantially liquid-impervious seal is formedbetween the outer sheath and the inner tank around the aperture. Spacingmeans is positioned intermediate the inner tank and the outer sheath toprovide for substantially free passage of liquids between a substantialportion of the exterior surface of the inner tank and at least a portionof the outer sheath.

The method of the present invention includes the steps of providing arigid inner tank having generally cylindrical sidewall portions, endportions general transverse to the sidewall portions and a central axisextending generally longitudinally between the end portions, with theinner tank having an exterior surface and at least one aperture formedthrough the inner tank. A first thermoplastic synthetic resin materialis applied to a portion of the exterior surface of the inner tanksurrounding and proximal to at least a portion of the aperture to definea layer of resin material surrounding and proximal to the aperture. Anouter sheath of a substantially liquid-impervious thermoplasticsynthetic resin material is applied over the inner tank and thesynthetic resin layer, with spacing means being provided intermediatethe exterior surface of the inner tank and the outer sheath to providefor substantially free passage of liquids between a substantial portionof the exterior surface of the inner tank and at least a portion of theouter sheath. The outer sheath is then bonded to the resin layersurrounding and proximal to the aperture, such that a substantiallyliquid-impervious seal is formed between the outer sheath and the outertank around the aperture.

BRIEF DESCRIPTION OF THE DRAWINGS

Particularly preferred embodiments of the method of the presentinvention will be described in detail below in connection with thefollowing drawings in which:

FIG. 1 is a side elevation, partially in section, of a tank according tothe present invention, illustrating various steps in the fabricationprocess;

FIG. 2 is a fragmentary upper perspective view of the tank of FIG. 1,illustrating the layer of material surrounding the apertures;

FIG. 3 is a fragmentary sectional view taken along line 3--3 of the tankof FIG. 2, illustrating the various layers of material applied in thefabrication process;

FIG. 4 is a fragmentary sectional view similar to FIG. 3, illustratingan alternative preferred embodiment with an additional layer of materialapplied proximal the aperture;

FIG. 5 is a fragmentary sectional view similar to FIG. 4, illustratingyet another alternative preferred embodiment of with an additional layerof material applied proximal the aperture.

FIG. 6 is a fragmentary sectional similar to FIG. 5, illustratinganother alternative preferred embodiment;

FIG. 7 is a side elevation, partially in section, of an alternativepreferred embodiment of the tank of FIG. 1, illustrating the steps inmaking it;

FIG. 8 is a fragmentary sectional view taken along lines 8--8 of thetank of FIG. 7 illustrating the overlapping extruded layers;

FIG. 9 is a fragmentary upper perspective view of an alternativepreferred embodiment of the tank of FIG. 1 with an outer sheathoverlaying apertures in the inner tank;

FIG. 10 is an upper perspective view of the tank of FIG. 9 with aportion of the outer sheath removed;

FIG. 11 is a fragmentary sectional view taken along line 11--11 of thetank of FIG. 10;

FIG. 12 is a top plan view of the tank of FIG. 13 illustrating thematerial surrounding the apertures;

FIG. 13 is a fragmentary sectional view taken along line 13--13 of thetank of FIG. 12 illustrating the various layers of material applied inthe fabrication process;

FIG. 14 is a fragmentary sectional view similar to that of FIG. 13,illustrating an alternative preferred embodiment;

FIG. 15 is a fragmentary sectional view similar to FIGS. 13 and 14illustrating yet another alternative preferred embodiment; and

FIG. 16 is an end sectional view of a completed tank according to FIG.1.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Preferred embodiments of the apparatus of the present invention areillustrated in FIGS. 1-16 in which similar reference numbers refer tocorresponding elements in the various views. FIG. 1 illustrates a tankfabricated according to a preferred method of the present invention, inwhich is provided a rigid inner tank, generally indicated as 10, havinggenerally cylindrical sidewall portions 12, closed end portions 14 and16, which are generally transverse to the sidewall portions 12.Elongated spindle members 18 and 20 are attached to and extend outwardlyof each of the end portions 14 and 16, respectfully, generally coaxiallywith the central axis of the inner tank 10, illustrated as referencenumber 22. As illustrated in FIG. 1, the spindle members 18 and 20 maybe supported by fixed supports 24 and 26 upon which the inner tank 10may conveniently be rotated about its axis 22, such as by a motor or anyconventional means, to facilitate the fabrication of the multiple wallstorage tank. Alternatively, rather than having spindle members 18 and20 attached to the end portions 14 and 16 of the inner tank 10, two ormore straps, such as continuous belts, may be positioned around aportion of the tank sidewall portions 12. The straps would then besimultaneously advanced in a conventional manner in order to rotate thetank 10 generally about its longitudinal axis 22. Still another approachwoud be to position inner tank 10 on a series of rollers to rotate innertank 10 generally about its axis 22.

While various forms and shapes of tanks may be utilized in practicingthis invention, the most common shape utilized for such storage tanks isthat of a cylinder, generally a right circular cylinder, having closedend portions 14 and 16. For simplicity of illustration thisconfiguration of tank is utilized for illustrating a preferredembodiment of this invention. Also, while virtually any construction ofrigid inner tank 10, whether metal or fiberglass or other materials, maybe utilized in practicing this invention, one preferred and readilyavailable type of structure is a tank formed of welded steel, having anappropriate corrosion resistant coating on its surfaces. Although theinner tank may have a corrosion resistant coating, it is not strictlynecessary because a later applied outer sheath acts as a corrosionresistant barrier. For purposes of illustration such a steel tank 10will be described. It is also to be understood that the tank 10 may be anewly fabricated tank, which may or may not have a manway opening orfittings cut in it, or it may be a previously used tank removed from itsprior installation and cleaned for remanufacturing in accordance withthis invention.

To prepare an uncoated or previously used steel inner tank 10, it isdesirable that the exterior surface of the tank be conventionally gritblasted and it may even be coated with a rust inhibitive material. It ispreferred that the spindle members 18 and 20 may be attached, suitablyby welding, to the center of the end portions 14 and 16, generallycoaxial with the tank axis 22, as set forth above. While the presentinvention will be described as a series of steps, it will be understoodand appreciated by those of ordinary skill in the art that the order ofvarious steps may be modified without departing from the spirit andscope of this invention, unless otherwise indicated. Accordingly, arigid inner tank 10 is provided that has at least one aperture, suitablya manway, indicated by reference number 28 on FIGS. 1 and 2, or aplurality of apertures, including manway 28, and fittings 30 and 32,formed through the sidewall portions 12. Apertures 30 and 32 areillustrated as fittings that include interior threads, which mayconveniently be connected to conventional piping for adding liquids toor withdrawing liquids from tank 10. During the fabrication process, itmay be desirable to cover or close such apertures, such as by welding anappropriately configured sheet of steel generally flush with the innertank exterior surface 17. It will become apparent that the method of thepresent invention may be applied to a new tank that does not includesuch apertures, and that appropriate such apertures may be formedthrough the tank late in the fabrication process. In order to simplifythis description, however, the preferred embodiment described hereinwill assume that the inner tank 10 includes at least one such aperture.

As shown in FIG. 2, a synthetic resin material 34, which mayconveniently be a thermoplastic resin material, such as a polyolefin,namely polyethylene or polypropylene, as well as other suitablematerials known in the art, is applied to a predetermined portion of theexterior surface of the inner tank surrounding and proximal to at leasta portion of the aperture 28 to form a layer of material 34 aroundaperture 28. Preferably, at least a portion of the resin layer 34 istreated, such as being oxidized, in a conventional manner to promote itsadhesion to the inner tank 10. Additional resin layers 36 and 38 arealso conveniently applied to the inner tank 10 around apertures 32 and34 in a similar manner. Where the inner tank 10 includes no apertures,the resin layer or layers should be applied to the inner tank exteriorsurface 17 at a predetermined position through where such apertures maybe formed subsequent to the application of the outer sheath. It will beunderstood and appreciated that, while three individual resin layers 34,36 and 38 are illustrated in the figures, a unitary resin layer may alsobe used, such as where the apertures 28, 30 and 32 are sufficientlyproximal one another.

In a preferred embodiment shown in FIG. 2, a portion, preferably atleast the lower surface, of each layer 34, 36 and 38 is convenientlytreated, such as being oxidized, to produce a receptive polar surface topromote the adhesion of the respective resin layers 34, 36 and 38 to theexterior surface of the inner tank 10. Numerous suitable methods ofsurface treatment are available for providing appropriate bondingbetween the tank 10 and the resin layers 34, 36 and 38, such as, forexample, flame treatment and electrical treatment, including plasmaetching, as well as others known in the art.

One preferred method for applying the resin layers 34, 36 and 38 to theinner tank 10 includes providing preformed patches of a suitablethermoplastic resin material, such as a polyolefin material, with atleast a portion of each respective patch positioned proximal to andsurrounding its adjacent aperture 28, 30 and 32, as shown in FIG. 2,with the opening formed through each patch aligned generally with itscorresponding aperture. While such patches are shown to have openingsformed through each respective patch that generally correspond to thesize and shape of apertures 28, 30 and 32, such patches may also begenerally solid sheets, with the apertures being cut through the sheetslater in the fabrication process.

When treating patches by a plasma etching process, the process may beperformed on a batch or a continuous basis, depending upon theparticular equipment involved. Suitable plasma processing equipmentincludes, for example, the B Series commercially available from AdvancedPlasma Systems, Inc. of St. Petersburg, Fla., among other models andvendors. A benefit of plasma treatment is that only the surface of thematerial is affected. Thus, the material properties beneath the surfaceof the patch remain substantially unchanged.

Once the patch 40 has been appropriately treated, the patch innersurface 52 may be directly bonded to the inner tank exterior surface 17,as shown in FIG. 3. Typically, a layer of an adhesive material 53,suitably an epoxy resin, may be interposed between the inner surface 52of patch and the inner tank exterior surface 17, as shown in FIG. 4.Then, by urging the inner surface 52 of patch 40 and the inner tankexterior surface 17 into engagement with the adhesive material 53, thepatch 40 is appropriately secured to the inner tank exterior surface 17surrounding aperture 28. In addition, it may be preferable to remove atleast a portion of the outer surface 54 of patch 40, suitably bysanding, grinding or other appropriate processes known in the art, inorder to remove the oxidized outer surface layer to facilitate theadhesion between a subsequently applied outer sheath 56 and the patchsecond surface 54.

It may be desirable to coat the treated surface 52 of the patch 40 withan appropriate adhesive, such as an epoxy resin or other materials knownin the art, soon after such treatment. In general, this reduces thepossibility of the surface treatment becoming contaminated or otherwiseineffective due to, for example, environmental exposure. In order tofacilitate the attachment of such a patch 40, the adhesive coatedsurface may be modified, suitably through light grit blasting orchemical cleaning, just prior to attaching to the inner tank exteriorsurface 17 to which has been applied an additional adhesive material,suitably an epoxy resin. The outer surface 54 is preferably untreatedor, if treated, the surface treatment may conveniently be removed topromote bonding to the thermoplastic synthetic resin outer jacket thatis to be applied subsequently, as described herein. Alternatively,rather than removing the surface treatment or oxidation from the outersurface 54, a protective film or mask may be temporarily applied to theouter surface 54 before the surface treatment process, such thatsubsequent to the surface treatment, the mask may be removed to exposethe untreated and unoxidized outer surface 54.

An alternative preferred approach to applying resin layers 34, 36 and 38includes flame spraying an appropriate thermoplastic synthetic resinmaterial, such as polyethylene, which may be in powder form, directlyonto the inner tank exterior surface 17 to form a layer of materialaround the apertures. Suitable flame spray systems include the 124 FlameSpray System available from Plastic FLAMECOAT® Systems of Big Spring,Tex. Of course, other appropriate systems may also be obtained fromother vendors. Resin layers 34, 36 and 38 are flame-sprayed onto theinner tank exterior surface surrounding and proximal to at least aportion of the apertures 28, 30 and 32, respectively. In order tofacilitate the bonding of the flame-sprayed resin layer 34, 36 and 38,it may be preferable to heat the inner tank proximal the apertures 28,30 and 32 as the resin layers 34, 36, and 38 are flame-sprayed onto theinner tank exterior surface 17.

As shown in FIG. 5, another preferred embodiment for applying thethermoplastic synthetic resin layer 34 to the inner tank 10 includesforming a substantially monolithic layer of materials surrounding andproximal to aperture 28. This may be accomplished by heating the innertank exterior surface proximal to and surrounding the aperture 28 andapplying a layer of thermosetting material 53, such as an epoxy, apolyester or a phenolic material, to a portion of the inner tankexterior surface 17 surrounding and proximal to aperture 28. Preferably,thermosetting material 53 is a fusion bond epoxy having a thickness thatmay vary between about 3 mils to about 25 mils and which is applied inpowder form, with the heated substrate, namely the inner tank exteriorsurface 17, melting the powder into a substantially continuous andviscous layer of material surrounding aperture 28. Prior to applyingthermosetting material 53, it is preferable that the exterior surface 17be appropriately conditioned, which conveniently may include gritblasting, to facilitate bonding between thermosetting material 53 andthe inner tank exterior surface 17.

An adhesive material 55, which may be a thermoplastic synthetic resinadhesive material known in the art, and preferably a chemically modifiedpolyolefin adhesive powder, such as used in the DUVAL Coating System, isapplied to at least a portion, and suitably all, of the thermosettingmaterial 53, preferably before thermosetting material 53 cures, suchthat adhesive material 55 chemically bonds to thermosetting material 53.Next, a thermoplastic resin material 34, which may suitably be a powdergrade polyolefin material, such as polyethylene, polypropylene,polyvinyl chloride, NYLON or a coal tar material, is applied to thesubstantially viscous layer of adhesive material 53 surrounding aperture28. The particular selection of material 34 will vary depending on theintended effect as well as which materials are selected for materials 53and 55. Again, heating the substrate 17 causes the resin layer 34 tobecome viscous and enable it to chemically bond to adhesive layer 55when cooled. The thicknesses of materials 55 and 34 may also be variedto suit requirements, typically between about 5 mils and about 40 mils,although other thicknesses of material may suffice. Preferably, each ofthe three layers of thermosetting material 53, adhesive 55 and syntheticresin material 34 is applied in powder form that melts after contactingthe heated tank or the heated previously applied layer, thereby formingthree viscous layers that chemically bond to their respective adjacentmaterials when cooled to define a substantially composite layer betweeninner tank 10 and the subsequently applied outer sheath 56. The outersheath 56 may then be applied over the resin layer 34 in a conventionalmanner, such as stated herein. While it is preferable that each layer ofmaterial be applied in a powder form to its respective heated substrate,it will be understood and appreciated that other configurations of theappropriate materials, such as sheets of film and liquids, may be usedwith equal facility.

As shown in FIGS. 1,3,4 and 5, another feature of the present inventionincludes providing spacing element, indicated generally as 62,intermediate the inner tank exterior surface 17 and the subsequentlyapplied outer sheath 56 to effect substantially free passage of liquidsalong a substantial portion of the inner tank exterior surface 17. Thespacing element 62 may conveniently be formed of a perforate materialsuch as wire or thermoplastic mesh, illustrated as 64 in FIGS. 1, 3 and4. This may suitably be the type of mesh conventionally used inreinforcing concrete structures over which a layer of imperforatematerial, such as paper or a stretched film of a synthetic resinmaterial, may be applied. Examples of suitable spacing means of thistype are disclosed in U.S. Pat. Nos. 4,644,627 and 4,640,439 bothinvented by David T. Palazzo, the inventor of the present invention.Other suitable types of spacing material 64 are described in furtherdetail in U.S. Pat. Nos. 4,744,137, 4,780,947 and 4,780,946 by David T.Palazzo, the inventor of the present invention. Of course, otherembodiments of the spacing element 62 may also be used, so long as theyprovide for substantially free passage of liquids between a substantialportion of the inner tank exterior surface 17 and the outer sheath 56.Some alternative preferred spacing elements will now be described.

One such preferred alternative of spacing element 62 includes applying arelatively thin flexible film, suitably having generally smoothsurfaces, over the properly prepared inner tank exterior surface 17.This spacing element 62 is particularly desirable when a heated ormolten outer sheath 56 is applied, such as by extruding a plurality ofoverlapping layers, over the spacing element 62 and inner tank 10. Asuitable such film should have higher melting temperature than thetemperature of the outer sheath when applied, such that as a heatedouter sheath 56 is applied over and engages the film 62, the film 62wrinkles or forms a plurality of crinkles, as shown in FIG. 6, but doesnot melt, thereby providing ridges or grooves for the desiredsubstantially free flow of liquids between the outer sheath 56 and theinner tank 10. Such film also may bond to outer sheath 56. Preferably,the ridges or grooves are positioned generally circumferentially aboutinner tank 10 adjacent the inner surface of outer sheath 56. Theflexible film may, for example, be formed of an elongated sheet ofpolyester material such as MYLAR®. Preferably the film 62 is spirallywrapped around the tank 10 from end to end in partially overlappinglayers, suitably as the tank 10 is rotated about its axis 22. Film 62also may be co-extruded with outer sheath 56. Of course, film 62 mayalso be applied by other methods known in the art, such as by sprayingit on or laying it up in sheets.

In order to provide for the desired bonding between outer sheath 56 andresin layer 34 around aperture 28, sufficient surface area of resinlayers 34 should be left exposed to outer sheath 56 to permit engagementtherebetween around aperture 28. For example, where mylar film 62 isused, a portion of the film 62 that covers resin layer 34 surroundingaperture 28 should be removed. However, the film should be preventedfrom falling off of or disengaging from inner tank 10. This mayconveniently be accomplished by causing film 62 to attach to a portionof tank outer surface 17, to a portion of resin layer 34 or to both.proximal the periphery of resin layer 34 around the aperture 28, such asby applying an adhesive material, which may include a double sidedmastic or adhesive strip, an adhesive liquid or other known adhesiveproducts, proximal to the resin layer periphery around aperture 28. Ofcourse, rather that removing film 62 from the resin layers outersurfaces, film 62 may be applied so as to leave at least a portion ofsuch surface area exposed, such that outer sheath 56 may engage theresin layers 34, 36 and 38.

As shown in FIG. 1, the next step includes applying an outer sheath 56of a substantially liquid impervious thermoplastic synthetic resinmaterial, such as a polyolefin, namely polyethylene, polypropylene, athermosetting resin, a catalyzing resin or other appropriate syntheticresins, over the inner tank 10 and the resin layers 34, 36 and 38.Preferably, at least a portion of the outer sheath 56 is spaced from atleast a portion of the inner tank exterior surface 17, suitably at leastthe lower portion, by spacing element 62 as described above.

Outer sheath 56 may conveniently be applied as a single sheet of athermoplastic synthetic resin material, such as a polyolefin, namelypolyethylene, that is wrapped around inner tank 10, spacing material 62and resin layers 34, 36 and 38. The sheet may conveniently be apre-extruded sheet of polyethylene that is wrapped tightly around theinner tank, resin layers and spacing material with the ends of the sheetbeing welded together laterally along the length of the tank. This typeof outer sheath also may conveniently be formed of a glass fiber resinmaterial similar to that disclosed in U.S. Pat. No. 4,644,627 to DavidT. Palazzo. A sheet of pre-extruded polyethylene is preferablysufficiently flexible to wrap around the cylindrical inner tank 10, andmay conveniently have a thickness of about 0.125 inch to about 0.25inch. After the outer sheath 56 is tightly wrapped around inner tank 10its ends secured, such as by welding, and it may then be bonded to theresin layers as is described in greater detail below.

FIGS. 7 and 8 show an alternative embodiment where the outer sheath 56is helically extruded by extruder 66 in situ over inner tank 10 andresin layers 34, 36 and 38 as a plurality of partially overlappingstrips or layers, which may suitably be heated to a molten resin ofabout 150 to about 600 degrees Fahrenheit, to facilitate bonding betweenthe plurality of partially overlapping layers. Such an extrusion processmay be similar to that shown and described in U.S. patent applicationSer. No. 08/735,610, the detailed description of which is incorporatedherein by reference. Outer sheath 56 may be extruded by a conventionalextruder 66, such as, for example, those commercially available fromGENCA of Clearwater, Fla., onto inner tank 10 as it is rotated. Ofcourse, extruders from other vendors may also be used. In addition, whena film, as described herein, is used as spacing element 62, applyingouter sheath 56 over film 62 conveniently causes spacing element 62 tocrinkle, shown in FIG. 6, defining a plurality of wrinkles andincreasing the flow of liquids. Each successive overlapping layer 73 ofouter sheath 56 has a predetermined width that is substantially lessthan the axial length of the cylindrical sidewall portions of inner tank10, suitably about two inches to about twelve inches. Each overlappinglayer 73 also includes an inner surface 70 and a leading edge 72.Preferably, the overlapping layers 73 are extruded such that at leasthalf, and suitably about four-fifths of width of each layer overlaps thepreceding layer, shown in FIG. 8.

Preferably, the plurality of partially overlapping layers 73 areextruded such that the intersection of the overlapping layers 73 form acorrugated inner surface of outer sheath 56 defining a plurality ofchannels 77 intermediate the outer sheath and the inner tank suitablyadjacent the inner tank exterior surface 17. These channels 77preferably extend adjacent the sidewall portions 12 generallycircumferentially about inner tank 10 between the inner tank and theouter sheath 56 to provide for desired substantially free flow ofliquids between the inner tank and the outer sheath as well as increasethe stiffness of the outer sheath in the radial direction with respectto axis 22. A portion of the corrugated inner surface, or a materialattached thereto, such as film 62 of FIG. 6, may engage the inner tankexterior surface 17, still allowing the substantially free flow ofliquids within channels 77. By modifying the amount of overlap betweenthe extruded layers 73 of outer sheath 56, the number of channels 77 mayconveniently be increased or decreased, with a corresponding change inthe rigidity of outer sheath 56. In addition, spacing elements 62, suchas those described herein, may be used in combination with the pluralityof channels 77 formed by the overlapping layers 73 to increase the freeflow of liquids between the inner tank 10 and the outer sheath 56.

As illustrated in FIGS. 7 and 8, rather than applying a spacing material64 as set forth above, the spacing element 62 may be providedintermediate inner tank exterior surface 17 and outer sheath 56 byforming protrusions, which may also be ridges, projections or grooves,indicated as 78, on the inner surface 70 of at least a plurality, andpreferably all of, those partially overlapping extruded layers. Suchprotrusions 78 are preferably formed proximal the leading edge 72 ofeach layer such that at least a portion of the protrusions 78 of eachoverlapping layer engage the inner tank exterior surface 17. In order toform such protrusions 78, appropriate modifications may be made to theextrusion die of the extruder 66 or, in the alternative, the extrudedpolyethylene material may be fed from the extruder 66 through a seriesof rollers appropriately configured to form the protrusions 78. In thismanner, the step of providing spacing element 62 is effectively combinedwith the extrusion step by the formation of the ridges 78, stillproviding the desired substantially free passage of liquids between theouter sheath 56 and at least a portion of the inner tank exteriorsurface 17. These ridges 78 may conveniently be applied to outer sheath56 in addition to the channels 77 formed by overlapping layers 73 tofurther provide for substantially free flow of liquids between innertank 10 and outer sheath 56. Of course, such protrusions may also beformed on the inner surface of the preformed sheet of outer sheath 56.Spacing element 62, may also comprise a release material, applied to theinner tank exterior surface prior to extruding outer sheath 56 toinhibit bonding between the outer sheath 56 and the inner tank exteriorsurface 17, further increasing the flow of liquids there between.

The next step is bonding the outer sheath to at least a portion of theresin layers 34, 36 and 38 surrounding the apertures 28, 30 and 32,respectively, such that a substantially liquid-impervious seal is formedaround each such aperture. It may be desirable to heat the inner tankproximal the apertures 30, 32 and 34 as the outer sheath 56 is beingapplied to promote adhesion or bonding between the respective resinlayers 34, 36 and 38 and the outer sheath 56. This may conveniently beaccomplished by attaching a heating member (not shown), such as bymagnetically attaching or clamping, to the inner tank 10 proximal theapertures 28, 30 and 32. For example, when outer sheath is extruded, asthe extruder approaches an aperture to which the heating member isattached, the heating member is disengaged from the inner tank to permitthe extruder to extrude the overlapping successive layers 73 of thepolyethylene sheath around the tank 10 and the apertures 28, 30 and 32as the tank is rotated. As stated above, it may also be desirable toposition a temporary cover over apertures or fittings, suitably bywelding or clamping. Alternatively, or in addition to applying heatproximal the apertures during the application of outer sheath 56,bonding between outer sheath 56 and resin layers 34, 36 and 38, and thusthe seal around fittings and manways, may be enhanced by heating theouter sheath proximal or surrounding the apertures subsequent to theapplication of outer sheath 56 to bond the outer sheath 56 to at least aportion of the resin layers 34, 36 and 38. This may conveniently beaccomplished by applying a heated plate or blanket or the like to theouter sheath proximal to the resin layers surrounding the apertures, orby other known methods.

Rather than applying a layer of synthetic resin material to the innertank exterior surface 17 prior to applying the outer sheath 56, FIGS.9-15 illustrate alternative preferred approaches, wherein the outersheath 56 is applied to inner tank 10 before the resin layers areapplied around the apertures 28, 30 and 32. In these alternativeembodiments, the outer sheath 56 is formed by applying a substantiallyliquid impervious thermoplastic synthetic resin material, as describedin greater detail above. After the outer sheath 56 has been applied tothe inner tank 10, a portion of the outer sheath proximal to andsurrounding each of the apertures 28, 30 and 32 is removed. This mayconveniently be accomplished by cutting the outer sheath proximal andsurrounding the apertures 28, 30 and 32, to define a cut in outer sheath56 that is spaced from and surrounding each aperture. The portion of theouter sheath 56 within each cut may then be urged away from the innertank exterior surface 17 to define openings 90, 92 and 94 through theouter sheath, as shown in FIG. 10. In addition, these cuts may also bemade in outer sheath 56 prior to its application, such as when a singlesheet outer sheath 56 is used. Each opening has a periphery, indicatedas 96, 98 and 100, surrounding each respective aperture 28, 30 and 32.The removal of the portion of the outer sheath surrounding the apertures28, 30 and 32 may be facilitated by a non-adhesive interface between thesynthetic resin outer sheath 56 and the typically metallic inner tank. Arelease material, which may conveniently be spacing element 62 or otherknown release materials, such as, for example, wax, grease, silicone,polyvinyl acetate (PVA), polyolefins in the form of mesh, nets or films,dust or powders, among others, may be applied to inner tank 10 to ensureouter sheath 56 does not permanently bond to the inner tank 10 as wellas provide for substantially free flow of liquids between inner tank 10and outer sheath 56.

A layer of a thermoplastic synthetic resin material, which mayconveniently be a polyolefin, such as polyethylene, polypropylene, orother conventional synthetic resins, is applied to a portion of theinner tank exterior surface 17 surrounding and proximal to at least aportion of each aperture 28, 30 and 32. Preferably, each resin layer isformed of a sheet or a patch 102, 104 and 106 having a central openingformed through the respective sheet 102, 104 and 106 with each openingbeing dimensioned according to the corresponding inner tank aperture 28,30 and 32 adjacent to which it is to be applied. In addition, each sheet102, 104 and 106 is preferably dimensioned, configured and positionedfor overlapping the respective opening peripheries 96, 98 and 100 thatare formed through the outer sheath 56. A suitable adhesive interface isprovided between each resin layer sheet 102, 104 and 106 and the innertank exterior surface 17 to promote the adhesion between the inner tankexterior surface 17 and the sheets.

FIG. 13 illustrates a preferred embodiment of the application of sheet102 in which an adhesive layer 108 is affixed to and integral with aninner surface of sheet 102 that is applied to aperture 28. Preferably,sheet 102 is formed of a sheet of cross linked polyolefin material thatis coated with a high shear strength, hot melt adhesive, such as iscommercially available as the Canusa Repair Patch (CRP) from Canusa ofOntario, Canada. Of course, other thermoplastic resin sheets andadhesives may be used with the present invention without departing fromthe scope of this invention. In this preferred embodiment, the adhesivematerial 108, suitably a hot melt adhesive, should be capable of formingrelatively strong and substantially permanent seals to conventionalthermoplastic synthetic resin materials, such as polyethylene andcertain epoxy and polyester materials, as well as to certain metals,including steel.

In order to apply sheet 102 to the appropriately prepared tank aroundaperture 28, the hot melt adhesive 108 is heated and then urged intoengagement with the inner tank exterior surface 17 surrounding andproximal to aperture 28, with the perimeter of sheet 102 overlapping theperiphery 96 of opening 90 in the outer sheath 56, shown in FIG. 13. Thearea of the inner tank 10 and outer sheath 56 to which the sheet 102 isto be applied may be preheated, suitably with a propane torch or otherknown heating instruments, to further promote bonding between the sheet102 and the tank 10 and outer sheath 56. In addition, after sheets 102,104 and 106 are applied to the tank as described above and shown withrespect to FIG. 13, the outer sheath is preferably bonded, suitably byheat welding, to those resin sheets, preferably proximal to theperimeter portions of the respective thermoplastic resin sheets 102, 104and 106 that overlap the outer sheath 56 surrounding the aperture. Ingeneral, this bonding forms the desired substantially liquid imperviousseal around each respective aperture. Accordingly, the spacing means 62provided between the outer sheath 56 and the inner tank 10 enablessubstantially free flow of liquids there between and the seal preventssuch liquids escaping from this interstitial space.

To further promote bonding between the resin sheets 102, 104 and 106 andthe inner tank exterior surface surrounding the respective apertures 28,30 and 32, an additional adhesive material 110, such as an epoxy resin,may be applied to the inner tank exterior surface surrounding andproximal to each respective aperture. This is shown in FIG. 14 withrespect to aperture 28, and this approach may be conveniently utilizedwith respect to each aperture 28, 30 and 32 of the tank. While, forpurposes of illustration, the additional adhesive layer 110 isillustrated as comparably thick as the outer sheath 56, it will beunderstood and appreciated that the thickness of adhesive layer 110 willtypically, although not necessarily, be substantially less than that ofouter sheath 56.

FIG. 15 illustrates yet another alternative approach for sealing theouter sheath 56 around each of the apertures 28, 30 and 32. In thisembodiment an inner patch 112 of a thermoplastic synthetic resinmaterial, such as, for example, polyethylene or polypropylene, ispositioned adjacent to the inner tank exterior surface 17 surroundingand proximal to at least one, and suitably each, of the apertures 28, 30or 32. Patch 112 is substantially similar to that illustrated anddescribed with respect to FIGS. 3 and 4, with the inner and/or outersurfaces appropriately treated, such as by one of the methods describedabove, to promote adhesive bonding between the inner surface of thepatch 112 and the inner tank exterior surface 17. Patch 112 is appliedto the inner tank exterior surface prior to the extrusion of the outersheath 56. A portion of the outer sheath proximal to and surroundingaperture 28 is removed, to define an opening in outer sheath 56 having aperiphery intermediate the inner patch inner boundary surrounding thepatch opening and the inner patch outer periphery. Thus, at least aportion, preferably a substantial part, of the outer surface of innerpatch 112 is visibly exposed to an observer.

Substantially as described with respect to FIGS. 3 and 4 and asillustrated in FIG. 15, an adhesive material 114 and 116, suitably anepoxy resin, may be applied to the inner and outer surfaces,respectively, of patch 112 in order to promote adhesive bonding betweenthe patch 112 and the inner tank exterior surface 17 and between theinner surface of sheet 102 and the outer surface of the patch 112.Adhesive layer 108 provides an adhesive interface between the sheet ofsynthetic resin layer 102 and the adhesive layer 116 of patch 112. Thus,by using the preferred combination of sheet 102 and adhesive layer 108,as described above, the adhesive layer may be heated and the adhesivelayer 108 of sheet 102 urged into engagement with the adhesive material116 of patch 112 to bond inner patch 112 to sheet 102 and to outersheath 56, thereby forming a substantially liquid impervious seal andsubstantially monolithic layer between the outer sheath and the innertank 10 surrounding aperture 28. A substantially identical process mayalso be used for apertures 30 and 32. As described above, an appropriatespacing means 62, which may conveniently be formed on the inner surfaceof outer sheath 56 or formed of a separate material, provide forsubstantially free passage of liquids between a substantial portion ofthe inner tank exterior surface 17 and a portion of the outer sheath 56.

To form the end portions 82 and 84 of the outer sheath 56, the syntheticresin material may simply be applied to the end portions, as shown inFIG. 1, or it may be extruded, as shown in FIG. 7, over the inner tankend portions 14 and 16 with or without appropriate spacing means. Forthe example when spindle members are employed to support inner tank 10and outer sheath 56 is extruded, the outer sheath may conveniently beapplied from spindle member 18 to spindle member 20, forming acontinuous jacket structure over the cylindrical sidewalls 12 of innertank 10 and end walls 14 and 16. To complete the end portions 82 and 84most of the length of each such spindle member 18 and 20 is then removedfrom each respective end portion 14 and 16 and a patch of asubstantially liquid-tight synthetic resin material, suitablypolyethylene, is attached, such as by welding, to the end portionscovering the area from where the respective spindle members 18 and 20were removed.

Alternatively, the end portions 82 and 84 may be fabricated separatelyfrom the cylindrical wall of the outer sheath. This method of formingthe end portions 82 and 84 separately may simplify the manufacturingprocess by permitting the formation of those end caps 82 and 84 over amale or female mold plug that would be appropriately secured and bondedto the extruded outer sheath 56 by welding or any other manner known tothose skilled in the art. While the end caps 82 and 84 are illustratedin the drawings as substantially flat end caps, it will be appreciatedand understood by those skilled in the art that end caps 82 and 84 mayalso be preformed cup or dome-shaped end caps. In addition, the end capsmay conveniently be attached to the inner tank prior to applying theouter sheath 56, such that when it is applied, the outer sheath willsubstantially conform to the shape of the end cap. Thus, by providing athermoplastic resin layer on the preformed end caps 82 and 84, thebonding between the end portions of the outer sheath and the end capsmay be facilitated.

As shown in FIG. 16 the completed cross sectional view of a preferredembodiment of the tank may include a pipe or tube 86 extending throughthe tank itself for use in detecting the presence of liquids or a changein pressure in the volume between inner tank 10 and the outer sheath 56.The tube 86 may be installed into aperture 30 or 32, which suitably mayinclude a threaded fitting. The tube 86 is inserted through the outersheath 56 and aperture 30 and conveniently extends through and is sealedto an aperture formed in the lowermost portion of inner tank 10 so thatthe lower portion of tube 86 is positioned between the inner tank 10 andthe outer sheath 56. Substantially, liquid-tight joints between the tube86 and the inner tank 10 are formed in any conventional manner, such asby welding to the inner tank 10. The tube 86 may conveniently extendinto the interstitial space between the outer sheath 56 and the innertank 10 to test for fluids, such that liquid disposed within theinterstitial space may conveniently be contacted by and withdrawnthrough the tube 86.

The remaining steps involved in the manufacture of the tank of thepresent invention generally depend upon the nature of the inner tank 10used in manufacturing multiple wall storage tank of the presentinvention. In general, if not already done, appropriate holes may be cutby any suitable means, such as a hole saw or the like, to define orreform apertures 28, 30 and 32. Then, suitable piping and a cylindricalmember 88, such as for a man way, may be appropriately secured, suitablyby welding or threading to the apertures 28, 30 and 32 of the inner tank10. In instances where patches and/or sheets have been affixed to theinner tank surrounding such apertures as described above, the piping andman way may conveniently be bonded to the respective layer ofthermoplastic resin material surrounding and proximal to the apertures28, 30 and 32 such as by heat welding or by applying an appropriateadhesive material to the interface between the cylindrical member 88 orpiping and the thermoplastic resin layer surrounding such interface. Inaddition the piping or manway may be secured to the inner tank, such asby threading or welding. A cover plate 91 may conveniently be secured tothe upper flange of the cylindrical member 88 by a conventional means,such as bolts 93. When all the fittings and cover plates are secured tothe tank, an appropriate resin material 95 may be applied over the tankto yield a finished structure such as shown in the cross sectional viewof FIG. 16.

By the foregoing construction there is provided a multiple wall tankthat can be manufactured economically from a conventional single walltank, and even from a used tank that has previously been removed fromunderground storage use. This structure provides an outer sheath, whichmay be formed from a material that is free of tendency to rust orcorrode, substantially sealed to the inner tank around an aperture andat least a portion of which may be spaced from the inner tank exteriorsurface by a spacing means to define an interstitial space in which thecollection and detection of liquids may be determined, such as by simplydetecting the presence of liquids or a change in pressure within thatspace.

While the foregoing describes in detail several preferred embodiments ofthe present invention, it is to be understood that such description isillustrative only of the principles of the invention and is not to beconsidered limitative thereof. Because numerous variations andmodifications of the present invention will readily occur to thoseskilled in the art, the scope of this invention is to be limited solelyby the claims appended hereto.

What is claimed is:
 1. A multiple wall tank for the storage of liquids having an outer jacket bonded to an inner tank surrounding and proximal to an inner tank aperture, said tank comprising:a rigid, inner tank having generally cylindrical sidewall portions, end portions generally transverse to said sidewall portions and a central axis extending generally longitudinally between said end portions, with said inner tank having an exterior surface, and at least one aperture formed through said inner tank; an outer sheath of a substantially liquid-impervious thermoplastic synthetic resin material surrounding said inner tank; a layer of a first thermoplastic synthetic resin material bonded and extending between said inner tank and said outer sheath surrounding and proximal to said aperture, such that a substantially liquid-impervious seal is formed between said outer sheath and said inner tank around said aperture; and spacing means positioned intermediate said inner tank and said outer sheath to provide for substantially free passage of liquids between a substantial portion of said exterior surface of said inner tank and at least a portion of said outer sheath.
 2. A tank as in claim 1 further comprising:a thermosetting material bonded to a portion of said inner tank exterior surface surrounding and proximal to said aperture; and wherein said first thermoplastic synthetic resin layer is bonded between said thermosetting material and said outer sheath, thereby forming a substantially composite layer of materials intermediate said inner tank and said outer sheath to define a substantially liquid impervious seal around said aperture.
 3. A tank as in claim 2 further comprising:a second thermoplastic synthetic resin material bonded to at least a portion of said thermosetting material surrounding and proximal to said aperture; and wherein said first thermoplastic synthetic resin material is bonded between said second synthetic resin material and said outer sheath, thereby forming a substantially composite layer of materials intermediate said inner tank and said outer sheath to define a substantially liquid impervious seal around said aperture.
 4. A tank as in claim 1 wherein said outer sheath further comprises a plurality of partially overlapping layers of a substantially liquid impervious thermoplastic synthetic resin material extruded in situ over at least a substantial portion of said inner tank, each successive said partially overlapping layer having a width substantially less than the axial length of said inner tank sidewall portions, an inner surface, and a leading edge, with said outer sheath being detached from a substantial portion of said inner tank exterior surface to define an interstitial space formed between at least a substantial portion of said outer sheath and said inner tank, which interstitial space provides for substantially free passage of liquids between said outer sheath and said inner tank, such that said interstitial space may be monitored for leakage.
 5. A tank as in claim 4 wherein said outer sheath further comprises a polyolefin resin material.
 6. A tank as in claim 4 wherein said spacing means further comprises protrusions formed on said inner surface of at least some of said partially overlapping extruded layers proximal to said leading edge of each said partially overlapping layer, such that at least a portion of said protrusions of each said extruded layer engage said inner tank exterior surface, thereby providing for substantially free passage of liquids between said outer sheath and at least a substantial portion of said inner tank exterior surface.
 7. A tank as in claim 4 wherein said first resin layer further comprises a patch having a pair of surfaces and a generally central opening formed therethrough, said patch opening having a perimeter portion generally corresponding to the dimensions and configurations of said inner tank aperture, with the patch opening aligned generally with said inner tank aperture, and with at least one said surface of said patch bonded to said outer sheath surrounding and proximal to said aperture; andsaid tank further comprises a layer of an adhesive material bonded between at least one said surface of said patch and said inner tank exterior surface surrounding and proximal to said aperture, thereby forming a substantially liquid-impervious seal between said inner tank and said outer sheath around said inner tank aperture. 